1. Field of the Invention
The present invention relates to a blur correction technique for a captured image.
2. Description of the Related Art
A recent increase in the quality and zooming ratio of an image capturing device causes a problem in that a captured image is blurred by a camera shaking when capturing an image, which led to the wide use of image capturing devices with a camera shake correction (e.g. deblurring) function. Such an image capturing device with a camera shake correction function generally adopts an optical camera shake correction method to optically correct a camera shake using a gyro sensor configured to detect an angular velocity dependent on a camera shake and a driving device configured to control the relative positions of the lens and image sensor so as to cancel the camera shake.
An image capturing device is proposed that uses an electronic camera shake correction method (patent literature 1 (Japanese Patent Laid-Open No. 2006-074693)). This image capturing device generates an image in which the influence of a camera shake is suppressed by electronically aligning and superimposing a plurality of images after these images are captured at high shutter speed. The electronic camera shake correction method proposed in patent literature 1 does not require a gyro sensor and thus is advantageous in downsizing of the image capturing device and reduction of the manufacturing cost.
Further, a technique to encode opening/closing of a shutter at the time of exposure when one image is acquired, and correcting a blur by arithmetic processing using the flutter shutter information (patent literature 2 (U.S. Patent Application Publication No. 2007/0258706)) is proposed. This technique is called coded exposure. A blur distribution is measured using a gyro sensor or the like, and a point spread function (to be simply referred to as PSF) is estimated as the blur function of a point image on an image. The frequency characteristic of the PSF is calculated, and filtering is done using a filter having a characteristic opposite to the calculated frequency characteristic or a Wiener filter. This enables satisfactory blur correction.
However, the method proposed in patent literature 1 is premised on a captured image being free from camera shake, and images suffering camera shake cannot be accurately aligned with each other. When an image such as a night scene is captured, noise is readily applied to the captured image, and noise reduction requires a predetermined exposure time or longer. As the exposure time becomes longer, a captured image may more readily contain a camera shake component, resulting in insufficient alignment between images. In patent literature 1, even adding and synthesizing aligned images cannot satisfactorily correct a camera shake.
In patent literature 2, when the frequency response value of the PSF is small, the blur correction amount at this frequency becomes larger than those at other frequencies, failing in satisfactory blur correction. Especially when an image to be corrected contains noise, the noise is amplified, greatly degrading the corrected image. Since a non-exposure time period is generated during image capturing, the amount of noise applied to a captured image tends to increase. Noise in image capturing remains in a blur-corrected image (e.g. deblurred image).